[docs] Annotate split-domain setup

[daenney]

Description

Small docs improvement that annotate each line with which config option the value needs to match.

Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: [ ] -> [x]

If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.

[daenney]

This is what it looks like: https://gotosocial--2372.org.readthedocs.build/en/2372/advanced/host-account-domain/

I need to figure out if there's a better way to annotate code in mkdocs. But for now it'll have to do.